This application claims priority from German Application No. 100 15 309.7, filed on Mar. 28, 2000, the subject matter of which is hereby incorporated herein by reference.
1. Field of the Invention
The invention relates to rubber mixtures, a process for their preparation and their use.
2. Background Information
It is known that hydrolysable organofunctionalized silanes which have the ability to react with fillers containing hydroxyl groups, such as, for example, naturally occurring and synthetic silicates, carbonates, glasses and metal oxides, find use for surface modification or adhesion promotion in many fields of use. In the rubber-processing industry, organopolysulfanesilanes (D 21 41 159) are used as adhesion promoters between the filler and rubber (D 22 55 577). The best known representative of this substance class is bis-triethoxysilylpropyltetrasulfane, known under the trade name Si 69 from Degussa-AG. Such organofunctionalized silanes are either used as pre-modified fillers (D 34 37 473) or the surface of the silicatic filler is modified with the liquid silane in situ during preparation of the mixture. Liquid organofunctional silanes are also used as solid mixtures, which are fixed to carriers, for rubber technology. By building up a chemical bond between the silicatic filler and the rubber matrix, good rubber properties, such as, for example, high moduli, high abrasion resistances and low hysteresis losses, are achieved. The additional hydrophobizing effect of the silane manifests itself in particular in low mixture viscosities, which allow processing of the rubber mixture.
The use of mercapto-functionalized organosilanes according to the formula R1R2R3Sixe2x80x94R4xe2x80x94Sxe2x80x94H, wherein R1, R2, R3 can be, identically or independently of one another: C1 to C4 alkyl, C1 to C4 alkoxy, preferably R1=R2=R3=methoxy or ethoxy,
R4 can be C1 to C6 linear or branched alkylidene, preferably propyl, for rubber mixtures is known from U.S. Pat. No. 3,350,345 and F 2 094 859.
The mercapto-functionalized organosilanes show a higher coupling effectiveness compared with the organopolysulfanesilanes described above and can therefore be used in a significantly lower dosage. It is known that because of their very high scorch sensitivity (scorching) and consequently more difficult processability, the use of these silanes plays only a minor role in rubber technology (U.S. Pat. No. 4,002,594).
The high scorch sensitivity of these silanes can be improved significantly and reliable processing can thus be ensured by introduction of so-called protective groups for the mercapto function (D 2 035 778, WO 99/09036). The introduction of such protective groups reduces not only the scorch sensitivity but also the coupling yield, which must be compensated by a higher dosage. This is undesirable because of the high prices of such silanes.
EP 784 072 describes the use of a combination of the mercaptosilane and a short-chain functionalized silicone oil. The mercaptosilane function is shielded here such that the scorch sensitivity is reduced significantly. The silicone oil furthermore has a hydrophobizing action, which improves the processability of these mixtures. The limited availability and the high costs for the functionalized silicone oils are disadvantages of this process.
The use of mercaptosilane in combination with a short-chain alkylsilane, preferably methyltrimethoxysilane, is furthermore known from U.S. Pat. No. 4,474,908. Improved vulcanisation product data are indeed obtained in this combination, but the poor scorching properties and the deteriorated processability are disadvantages.
It is an object of the present invention to provide a rubber mixture which has outstanding processing properties (low viscosity, long scorch times) and exceptional vulcanization product data (e.g. high amplification ratio 300%/100% and low hysteresis loss).
The invention provides a rubber mixture, which is characterized in that this comprises
(a) a rubber or a mixture of rubbers,
(b) a silicatic filler,
(c) an organosilane of the general formula (I)
R1R2R3Sixe2x80x94R4xe2x80x94SHxe2x80x83xe2x80x83(I), 
wherein R1, R2, R3 can be identical or different and consist of C1 to C4 alkyl or C1 to C4 alkoxy, preferably methoxy or ethoxy,
R4 consists of C1 to C6 linear or branched alkylidene, preferably propyl, and
(d) an alkylsilane of the general formula (II)
R1R2R3Sixe2x80x94R5xe2x80x83xe2x80x83(II) 
xe2x80x83wherein R1, R2, R3 have the meaning as in formula I, R5 consists of C10-C20 linear or branched alkylidene, preferably hexadecyl or octadecyl.
Natural rubber and/or synthetic rubbers can be used as the rubber. Preferred synthetic rubbers are described, for example, in W. Hofmann, Kautschuktechnologie [Rubber Technology], Genter Verlag, Stuttgart 1980. They can comprise, inter alia,
polybutadiene (BR)
polyisoprene (IR)
styrene/butadiene copolymers with styrene contents of 1 to 60, preferably 5 to 50 wt. % (SBR)
isobutylene/isoprene copolymers (IIR)
butadiene/acrylonitrile copolymers with acrylonitrile contents of 5 to 60, preferably 10 to 50 wt. % (NBR)
ethylene/propylene/diene copolymers (EPDM)
and mixtures of these rubbers.
In a preferred embodiment, the rubbers can be vulcanizable with sulfur.
Precipitated silicas can be employed as the silicatic fillers.
The rubber mixtures can comprise 10 to 150 parts by wt. of silicatic filler, 0.02 to 4 parts by wt. of organosilane of the formula I and 0.02 to 10 parts by wt. of alkylsilane of the formula II, the parts by wt. being based on 100 parts by wt. of rubber.
The organosilane of the formula I can be employed in an amount of 1 to 4 wt. % and the alkylsilane of the formula II in an amount of 1 to 6 wt. %, based on the amount of filler employed.
The rubber mixtures according to the invention can comprise carbon black, for example furnace black, gas black, channel black, lamp black, thermal black, acetylene black, plasma black, inversion black, known from DE 195 21 565, Si-containing carbon black, known from WO 98/45361 or DE 196 13 796, or metal-containing carbon black, known from WO 98/42778, electric arc black and carbon blacks, which are by-products of a chemical production process. The carbon black can be activated by upstream reactions.
The rubber mixtures according to the invention can comprise further known rubber auxiliary substances, such as, for example, crosslinking agents, vulcanization accelerators, reaction accelerators or retardants, anti-ageing agents, stabilizers, processing auxiliaries, plasticizers, waxes, metal oxides and activators, such as triethanolamine, polyethylene glycol, hexanetriol.
The rubber auxiliary substances can be employed in conventional amounts, which depend, inter alia, on the intended use. Conventional amounts are, for example, amounts of 0.1 to 50 wt. %, based on the rubber.
Sulfur or organic sulfur donors can serve as crosslinking agents.
The rubber mixtures according to the invention can furthermore comprise vulcanization accelerators. Examples of suitable vulcanization accelerators are mercaptobenzothiazoles, sulfenamides, guanidines, thiurams, dithiocarbamates, thioureas and thiocarbonates. The vulcanization accelerators and sulfur can be employed in amounts of 0.1 to 10 wt. %, preferably 0.1 to 5 wt. %, based on the rubber employed.
The invention also provides a process for the preparation of the rubber mixtures according to the invention, which is characterized in that the rubber or the mixture of rubbers, the silanes according to the formulae I and II and the silicatic filler are mixed in a mixing unit and the silanes according to the formulae I and II are added together or separately in succession.
The mixing of the rubbers with the filler, optionally rubber auxiliary substances and the organosilanes can be carried out in conventional mixing units, such as roll mills, internal mixers and mixing extruders. Such rubber mixtures can conventionally be prepared in internal mixers, the rubbers, the filler, the organosilanes and the rubber auxiliary substances first being mixed in at 100 to 170xc2x0 C. in one or several successive thermomechanical mixing stages. The sequence of addition and the time of addition of the individual components can have a decisive effect on the resulting mixture properties here. The crosslinking chemicals can conventionally be added to the rubber mixture obtained in this way in an internal mixer or on a roll mill at 40-110xc2x0 C. and the mixture can be processed to the so-called crude mixture for the subsequent process steps, such as, for example, shaping and vulcanization.
The vulcanization of the rubber mixtures according to the invention can be carried out at temperatures of 80 to 200xc2x0 C., preferably 130 to 180xc2x0 C., optionally under a pressure of 10 to 200 bar.
The rubber mixtures according to the invention can be used for the production of shaped articles, for example for the production of pneumatic tires, tire treads, cable sheathings, hoses, drive belts, conveyor belts, roller coverings, tires, shoe soles, sealing rings and damping elements.
The invention also provides shaped articles obtainable from the rubber mixture according to the invention by vulcanization.
The rubber mixtures according to the invention have the advantage that the long-chain alkylsilanes effectively shield the mercapto function such that the scorching time is prolonged significantly, but the coupling effectiveness of the mercaptosilane is not reduced. As a result, the amount of mercaptosilane required can be set significantly lower than for conventional organopolysulfanesilanes. Furthermore, the use of the long-chain alkylsilane has the effect of a very good hydrophobization of the filler surface, which manifests itself in low mixture viscosities and therefore a very good processability.